1. Field of the Invention
This invention relates to a liquid crystal composition and a TN type liquid crystal display element using the same. More particularly it relates to a liquid crystal composition capable of improving reduction in the threshold voltage (hereinafter abbreviated often to Vth) of TN type liquid crystal display elements and the temperature dependency thereof.
2. Description of the Prior Art
In recent years, the performance of liquid crystal display devices have been improved and their fields of application have come to be further extended. Initially liquid crystal display devices were used mainly for handy type electric calculators, watches, clocks, etc., making use of their specific feature of low electrical power consumption. However, with development of liquid crystal compositions, the fields of application have recently come to be rapidly extended to those of large type display devices such as outdoor display devices, instrument panels for automobiles, display devices for hand-held computors or terminal equipments of computors, etc.
When this is viewed from the point of the drive mode of display devices, the 3 V drive static mode at the initial period is being transferred, for enlargement of the information content, reduction in the number of parts, etc., toward a high performance display mode having e.g. the following number of multiplex drive at a definite drive voltage:
1/2 duty at 3 V drive in the case of watches, clocks, etc., 1/3-1/4 duty at 3 V drive in the case of electric calculators, and 1/16 duty at 4.5 V drive and 1/64 duty or more at ten and several V in the case of word processors, computer terminal, etc. Further, in addition to such a development, requirement for low voltage drive which is another specific feature of liquid crystal display devices has been intensifying in order to make LCD smaller in size and lighter in weight. For example, there have been desired 1/2-1/4 duty at 1.5 V drive in the case of waters, clocks, electric calculators, etc., 1/16 duty at 3 V drive in the case of word processors, etc., and 1/64 duty at 10 V or lower voltage drive. It has been desired to reduce the threshold voltage with respect of the voltage-brightness characteristic of liquid crystal compositions, but in the case of conventional material systems, it has been very difficult to effect a low voltage drive while other characteristics such as ensuring the display grade, mentioned later, the temperature dependency of the threshold voltage and the viscosity relative to the response time are satisfied.
Usually, in the case of the multiplex drive mode employing the voltage-leveling method, there is existent in principle, an operational limitation which does not occur at all in the case of the static mode. The operational limitation arises from the occurrence of a crosstalk at half-selected or unselected elements. The crosstalk is more liable to occur in a higher multiplexed matrix addressing scheme where the operation margin i.e. the allowance of the operation voltage is reduced.
There are the following three factors on which the operational mergin depends:
(1) the temperature dependency of threshold voltage, PA1 (2) the view angle dependency of threshold voltage, and PA1 (3) the steepness of threshold voltage.
As to the factor (2) among these, the cell preparation conditions, such as the thickness of the display cell, have a great influence, and the difference between the material systems has a small influence. As to the factor (3), this is specific of TN type liquid crystal display devices and there occurs no notable difference which depends on the material systems. Whereas, as to the factor (1), the influence of the material systems is greatest among the factors in the aspect of its relation to the operation temperature range. In order to improve the factor (1), it has generally been carried out as a countermeasure directed to liquid crystal compositions to reduce the temperature dependency of the threshold voltage by increasing the proportion of a material having a negative dielectric anisotropy (hereinafter abbreviated often to .DELTA..epsilon.). However, an increase in the proportion of a material having a negative dielectric anisotropy (.DELTA..epsilon.) results in elevation of the threshold voltage so that it has been impossible to achieve a sufficiently low voltage drive.
Further, for the display devices, the response time is a non-negligible, important property. It has been well known that the response speed of liquid crystal display devices has a relationship to the viscosity of liquird crystal compositions relative thereto. When only conventional materials are used for achieving reduction in the threshold voltage, the resulting liquid crystal compositions have a high viscosity; hence it has been very difficult to achieve a satisfactory response time for the display devices.